From Engineering Services
Composting is an aerobic process, i.e. oxygen is needed for the process. The microbes that break the organic material down are especially bacteria and fungi. The work of the microbes depends very much on certain conditions. The access to oxygen, the carbon/nitrogen ratio, temperature, the structure of the material, pH value and the water content are examples of important parameters. If these parameters can be controlled you can optimise the breaking down process. If, for instance, the access of oxygen is insufficient the breaking down will stop. Instead an anaerobic process will start, fermentation, which will be noticed by the nasty-smelling gas that is produced. If the material is clean it is an advantage if it is disjointed, as the microbes will have a larger area for their work. On the other hand a disjointed material is packed together and therefore the access of oxygen will be more difficult.
Big scale composting takes place through one of two basic principles - open air composting or closed composting. The composting process can also be divided into pre treatment, pre composting, and post composting and post treatment. Both open air and closed composting can be equipped with technical equipment that is wanted.
As mentioned there are some disadvantages with fermentation and open air composting. Therefor the interest has been focusing reactor composting in a composting drum. With reactor composting the process can be controlled so the breaking down goes quickly and is more complete and the emissions are possible to measure and clean. A programme of quality control can guarantee the process and the high quality of the compost. The finished composed can be returned to producing soil in a controlled way.
During the 1970's and 1980's several reactor composting facilities were build. The incoming materials were crushed or grind. As a result unsuitable material from for instance batteries, cans wit insect repellents and paint were mixed with the organic material in the composting drum. The process in the drum, which was not divided into compartments, lasted for several weeks so some metals were dissolved and contaminated the compost. The sewage sludge often had a low quality and the household waste contained more unsuitable materials than today. These circumstances led to a low compost quality. Those effects are often used as argument against big scale composting even today. The quality of the incoming material has improved and the technology in a well designed composting facility is quit different from earlier reactor composting. This technology is developed during many years and is well proven.
The right composting method
When you are about to decide what technology to choose, it is important to decide how you look upon the value of the produced compost and in what way it may be used. It is a great difference if you think it is enough to use the compost as a layer on the landfill or as bio filter for the methane gas production at the landfill, or if you think that it may be used as fertiliser in the agriculture for food production where the quality of the compost is of utmost importance. In other words, it is essential with a high and reliable quality of the compost day after day so the customers always can rely on the product. To be able to produce a high quality compost you need a quality control system.
Another decision is whether sewage sludge or manure from pig or hen farms shall be co-composted together with the household waste. With our method you can compost with or without sewage sludge.
There are some different big scale composting technologies and on our homepage we have described a few of them with quite high level of the process because this is necessary if you want a useful compost.
Consequently it is important if you think that composting is an expensive way to get rid of most of the waste, as you reduce the volume very much, or if you think that composting is a chance to produce a valuable product which is absolutely necessary for productive soil. It should be obvious that the organic waste is a resource, which must be taken care of so nutrients and humus can be returned to agriculture and forestry in a controlled way.